Polishing method

ABSTRACT

A wafer is pressed against a fixed abrasive and brought into sliding contact with the fixed abrasive. Thus, the wafer is polished. A surface of the fixed abrasive is dressed so as to generate free abrasive particles thereon. A liquid or a gas, composed of a mixture of liquid or inert gas and pure water or chemical liquid, is ejected onto the surface of the fixed abrasive during or after the dressing process.

This application is a Divisional of U.S. Ser. No. 10/330,189, filed Dec.30, 2002.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a polishing method, and moreparticularly to a polishing method of polishing a workpiece such as asemiconductor wafer with a fixed abrasive.

2. Description of the Related Art

As semiconductor devices become more highly integrated in recent years,circuit interconnections have become finer and distance between thosecircuit interconnections becomes smaller. In case of photolithographywhich can form interconnections that are at most 0.5 μm wide, it isrequired that surfaces on which pattern images are to be focused by astepper should be as flat as possible because a depth of focus of anoptical system is relatively small. A polishing apparatus for performingchemical mechanical polishing (CMP) has been used for planarizing asemiconductor wafer.

This type of chemical mechanical polishing (CMP) apparatus comprises apolishing table having a polishing pad attached thereon, and a top ringfor holding a workpiece, to be polished, such as a semiconductor wafer.The workpiece is disposed between the polishing pad and the top ring andpressed against the polishing pad under a certain pressure by the topring while the polishing table and the top ring are being rotated. Theworkpiece is polished to a flat mirror finish while a polishing liquid(slurry) is being supplied onto the polishing pad.

When the aforementioned chemical mechanical polishing process iscontinuously performed, polishing particles or polishing wastes areattached to the polishing pad, resulting in a change in properties ofthe polishing pad and a deterioration in polishing performance.Therefore, if an identical polishing pad is repeatedly used forpolishing semiconductor wafers, problems such as lowered polishing rateand uneven polishing are caused. In order to overcome such problems,conditioning called dressing is performed before, after or duringpolishing of a semiconductor wafer to regenerate the polishing pad.

In a chemical mechanical polishing process using a polishing liquid asdescribed above, a workpiece is polished while a polishing liquidcontaining a large amount of abrasive particles is being supplied onto arelatively soft polishing pad. Therefore, a problem of patterndependence arises. Pattern dependence means that gentle irregularitiesare formed on a surface of a semiconductor wafer after a polishingprocess due to irregularities on the surface of the semiconductor waferthat existed before the polishing process, thus making it difficult toplanarize the surface of the semiconductor wafer to a completely flatsurface. Specifically, a polishing rate is higher in an area whereirregularities have small pitches (a density of irregularities is large)and is lower in an area where irregularities have large pitches (adensity of irregularities is small), and existence of areas of thehigher polishing rate and areas of the lower polishing rate causesgentle irregularities to be formed on the surface of the semiconductorwafer. Further, during the polishing process using the polishing pad,since not only convexities but also concavities of the irregularities onthe surface of semiconductor wafer are polished, it is difficult to stopthe polishing process when the convexities of the irregularities arepolished to a flat surface.

It has also been practiced to polish a semiconductor wafer with use of afixed abrasive (grindstone) which comprises abrasive particles of ceriumoxide (CeO₂) or the like fixed by a binder such as phenolic resin. Apolishing process utilizing the fixed abrasive is advantageous in thatpolishing material, i.e., the fixed abrasive, is harder than a polishingpad used in a conventional CMP process, and tends to polish convexitiesof the irregularities more than concavities thereof, for therebyachieving a higher absolute level of planarity. Depending on compositionof the fixed abrasive, the fixed abrasive provides a self-stop functionwhich considerably lowers a polishing rate and practically stops apolishing process when the convexities of the irregularities arepolished to a flat surface. The polishing process utilizing the fixedabrasive is also advantageous in that environmental load can be reducedbecause of no use of a suspension liquid (slurry) containing a largeamount of abrasive particles.

However, when a dressing process is performed on the aforementionedfixed abrasive, massive particles (masses of polishing particles) tendto be produced on a surface of the fixed abrasive. The massive particlesmay enter a boundary between the semiconductor wafer and the fixedabrasive to cause scratches to be produced on a surface of thesemiconductor wafer. After the semiconductor wafer is polished with thefixed abrasive, abrasive particles contained in the fixed abrasive areattached to the surface of the semiconductor wafer. Therefore, it isnecessary to prevent the semiconductor wafer from being contaminated bythe abrasive particles.

SUMMARY OF THE INVENTION

The present invention has been made in view of the above drawbacks. Itis therefore an object of the present invention to provide a polishingmethod which can effectively remove massive abrasive particles producedon a surface of a fixed abrasive, by performing a dressing process, toprevent scratches from being produced on a surface of a workpiece, andcan remove abrasive particles attached to a surface of a workpiece aftera polishing process to prevent the workpiece from being contaminated.

According to a first aspect of the present invention, there is provideda method comprising: polishing a workpiece by pressing the workpieceagainst a fixed abrasive and bringing the workpiece into sliding contactwith the fixed abrasive; dressing a surface of the fixed abrasive so asto generate free abrasive particles thereon; and ejecting (or atomizing)a liquid or a gas, composed of a mixture of liquid or inert gas and purewater or chemical liquid, onto the surface of the fixed abrasive duringor after the dressing of the surface of the fixed abrasive.

As described above, when the dressing process is performed on the fixedabrasive, massive particles (masses of polishing particles) tend to beproduced on the surface of the fixed abrasive. According to the presentinvention, atomization is performed on the surface of the fixed abrasiveduring the dressing process or immediately after the dressing process.Therefore, even if massive particles, which cause scratches on a surfaceof the wafer, are produced on the surface of the fixed abrasive by thedressing process, the atomization can remove the massive particles fromthe surface of the fixed abrasive to prevent the workpiece from beingscratched.

It is free fine abrasive particles present on the surface of the fixedabrasive that contribute to a polishing process of the workpiece. Thesefree fine abrasive particles are unlikely to be removed by atomization.Therefore, while massive particles are removed as described above, thefree fine abrasive particles which contribute to a polishing rate arenot removed, and the polishing rate is not affected by atomization.However, if pressure of an inert gas is higher than 0.5 MPa, then thefine abrasive particles are likely to be removed to thereby lower thepolishing rate. Therefore, it is desirable that a flow rate of theliquid is within a range of from 200 to 5000 cm³/min, and pressure ofthe inert gas is within a range of from 0.05 to 0.5 MPa. Morepreferably, the flow rate of the liquid is about 1000 cm³/min, and thepressure of the inert gas is about 0.15 MPa.

Such atomization may be performed in either case of an in-situ dressingprocess in which a dressing process is performed during a polishingprocess of a workpiece, or an ex-situ dressing process in which adressing process is performed when the workpiece is not polished. Theatomization should preferably be performed during the dressing process.Particularly, in the case of the in-situ dressing process, it isnecessary to perform the atomization during the dressing process.

The liquid should preferably be ejected toward an outer peripheral edgeof the fixed abrasive. When the liquid is ejected toward the outerperipheral edge of the fixed abrasive, the aforementioned massiveparticles can efficiently be removed from the surface of the fixedabrasive. When gas is ejected onto the surface of the fixed abrasive forremoving the massive particles, the liquid is supplied together with thegas onto the surface of the fixed abrasive. The gas is ejected onto theliquid supplied to the surface of the fixed abrasive to thereby promoteremoval of foreign matter from the surface of the fixed abrasive.

According to a second aspect of the present invention, there is provideda method comprising: polishing a workpiece by pressing the workpieceagainst a fixed abrasive and bringing the workpiece into sliding contactwith the fixed abrasive while dressing a surface of the fixed abrasive;and continuously polishing the workpiece while not dressing the surfaceof the fixed abrasive.

From a viewpoint of wear resistance, the dressing process shouldpreferably be performed with a dressing tool (diamond dresser) havingparticulates such as diamond particles electrodeposited thereon.

As described above, if massive particles are produced on the surface ofthe fixed abrasive by performance of the dressing process and enter theboundary between the workpiece and the fixed abrasive, then the massiveparticles are crushed to cause scratches to be produced on a surface ofa workpiece. According to the present invention, the workpiece ispolished while dressing a surface of the fixed abrasive, and then ispolished while not dressing the surface of the fixed abrasive.Therefore, even if massive particles are produced on the surface of thefixed abrasive by performance of the dressing process, the massiveparticles are crushed or removed from the surface of the fixed abrasiveduring the polishing process, so that scratches are not newly producedon the surface of the workpiece. Specifically, although scratches may beproduced on a surface of a workpiece during an initial stage of apolishing process, the scratches can gradually be shallowed bycontinuously polishing the workpiece and can finally be eliminated.Further, although scratches are continuously produced on a surface of aworkpiece during an in-situ dressing process, these scratches can beeliminated by stopping the dressing process and continuing a polishingprocess.

In a dressing process before a polishing process, i.e., an ex-situdressing process, when atomization is performed together with the abovemethod, scratches can be prevented more effectively. It is desirable tosupply pure water (DIW) during the dressing process, and ultra purewater or a chemical liquid containing no abrasive particles as apolishing liquid during the polishing process, onto the surface of thefixed abrasive.

If surface pressure during a dressing process is high, then the numberof massive particles (masses of polishing particles) produced becomeslarge, and larger massive particles are likely to be produced. As aresult, the number of scratches on the workpiece is increased, anddepths of the scratches become greater. Since such massive particleshardly contribute to improvement of a polishing rate, it is desirablethat the number of massive particles be small. Therefore, surfacepressure during the dressing process should preferably be set to be assmall as possible, 9.8 kPa or lower, for example. Further, it isdesirable that surface pressure of the workpiece, when the workpiece isbeing polished while not dressing the surface of the fixed abrasive, beset to be smaller than that when the workpiece is being polished whiledressing the surface of the fixed abrasive.

According to a third aspect of the present invention, there is provideda method comprising: polishing a workpiece by pressing the workpieceagainst a fixed abrasive and bringing the workpiece into sliding contactwith the fixed abrasive; and then water-polishing the workpiece whilesupplying pure water to the fixed abrasive, wherein surface pressure ofthe workpiece during the water-polishing is set to be smaller than thatduring the polishing using the fixed abrasive.

During the water-polishing process, abrasive particles attached to thesurface of the workpiece can be cleaned and removed from a surface ofthe workpiece. The water-polishing process should preferably beperformed for 5 seconds or longer.

A rotational speed of the fixed abrasive (polishing table) during thepolishing process is usually 30 revolutions per minute or smaller. Therotational speed of the polishing table should preferably be increasedto 50 revolutions per minute or higher to enhance an effect of cleaningand removing abrasive particles attached to the surface of theworkpiece.

When a fixed abrasive is used for polishing a workpiece, the workpiecemay be polished even during a water-polishing process. In order toprevent such a drawback, it is necessary to reduce surface pressure ofthe workpiece as small as possible. Although surface pressure of theworkpiece during the polishing process using the fixed abrasive isusually 29.4 kPa or higher, surface pressure of the workpiece during thewater-polishing process should preferably be set to be smaller than thatduring the polishing process using the fixed abrasive. Specifically, thesurface pressure of the workpiece during the water-polishing processshould preferably be reduced to 29.4 kPa or lower, more preferably 20kPa or lower.

Water-polishing should preferably comprise supplying pure water at aflow rate larger than a flow rate of a polishing liquid supplied duringpolishing using a fixed abrasive.

When a workpiece is separated or removed from a surface of the fixedabrasive, a portion of the workpiece is exposed beyond an outerperipheral edge of the fixed abrasive so as not to leave the wafer onthe surface of the fixed abrasive, which is called an overhangingprocess. However, if rotational speed of the polishing table is high,then the workpiece cannot stably be held at a overhanging position by atop ring so as to cause scratches or unevenly polished portions to beproduced on the workpiece by the outer peripheral edge of the fixedabrasive. Therefore, when the workpiece is lifted from the fixedabrasive (polishing table) during the overhanging process, rotationalspeed of the polishing table should preferably be reduced to 10revolutions per minute or lower. The workpiece should preferably beremoved directly from the surface of the fixed abrasive without theoverhanging process, in which a portion of the workpiece is exposedbeyond the outer peripheral edge of the fixed abrasive.

According to a fourth aspect of the present invention, there is provideda method comprising: polishing a workpiece by pressing the workpieceagainst a fixed abrasive and bringing the workpiece into sliding contactwith the fixed abrasive; and then cleaning (or buff cleaning) theworkpiece by pressing the workpiece against a soft cleaning surface andsupplying a liquid containing no abrasive particles to the cleaningsurface.

When the workpiece is polished with the fixed abrasive, abrasiveparticles contained in the fixed abrasive are likely to be attached to asurface of the workpiece immediately after the polishing process.Particularly, abrasive particles of ceria are easily attached to asurface of a silicon oxide film. According to the present invention,after the polishing process, the workpiece is pressed against a softcleaning surface, and a liquid containing no abrasive particles issupplied to the cleaning surface. With a cleaning process, the abrasiveparticles attached to the surface of the workpiece can be removed fromthe surface of the workpiece. Soft surface means a surface having alarge modulus of compression elasticity.

In this case, pure water may be supplied to the cleaning surface. It ismore effective to supply an alkali liquid, preferably an alkali liquidhaving a pH of 9 or larger because surface potential (zeta potential) ofceria and an oxide film becomes negative in an alkali region, and theceria and the oxide film become likely to become dissociated from eachother by repulsion. When the alkali liquid contains tetramethylammoniumhydroxide (TMAH), the ceria and the oxide film are more likely to becomedissociated from each other, so that an effect of removing the abrasiveparticles can be enhanced.

According to a fifth aspect of the present invention, there is provideda method comprising: polishing a workpiece by pressing the workpieceagainst a fixed abrasive and bringing the workpiece into sliding contactwith the fixed abrasive; and cleaning (or DHF cleaning) a surface of theworkpiece with dilute hydrogen fluoride after the polishing of theworkpiece.

When the workpiece is polished with the fixed abrasive, abrasiveparticles contained in the fixed abrasive are likely to be attached to asurface of the workpiece after the polishing process. In order to removethe abrasive particles attached to the surface of the workpiece, a DHFcleaning process may be performed instead of a buff cleaning process.When dilute hydrogen fluoride is added to a surface of a silicon oxidefilm, the oxide film is slightly dissolved. For example, when a DHFliquid of 0.5% is added to the surface of the silicon oxide film forabout 30 seconds, the oxide film is dissolved by a thickness of about 50Å. Thus, the oxide film is dissolved and removed together with abrasiveparticles attached to the surface of the oxide film. In this case, it isdesirable to use a DHF liquid having a concentration of 0.1% or higher.

When scrubbing of a workpiece by a roll sponge, a pencil-type sponge, ora brush is accompanied with this DHF cleaning, the abrasive particlescan be removed more effectively. It is desirable to perform the DHFcleaning process immediately after the polishing process of theworkpiece. However, the DHF cleaning process may be performed on aworkpiece which has already been dried.

In a polishing method of pressing a workpiece against a polishingsurface and bringing the workpiece into sliding contact with thepolishing surface to polish the workpiece, a polishing liquid containingabrasive particles may be supplied to the polishing surface after thepolishing process of the workpiece to perform a final polishing. In thiscase, the polishing surface may comprise a fixed abrasive or a polishingpad other than the fixed abrasive (for example, IC 1000 or POLITEX madeby Rodel Corp).

According to a sixth aspect of the present invention, there is provideda method comprising: polishing a workpiece by pressing the workpieceagainst a first polishing tool having a diameter and bringing theworkpiece into sliding contact with the first polishing tool, whilesupplying a chemical liquid containing no abrasive particles to thefirst polishing tool; and further polishing the workpiece by subjectingthe workpiece to a second polishing tool having a diameter larger thanthe diameter of the first polishing tool while supplying a polishingliquid containing abrasive particles to the second polishing tool.

According to a seventh aspect of the present invention, there isprovided a method comprising: polishing a workpiece with a fixedabrasive while supplying a chemical liquid to a surface of the fixedabrasive; and simultaneously ejecting at least one of a liquidcontaining the chemical liquid, and a fluid composed of a mixture ofinert gas and the chemical liquid, onto the surface of the fixedabrasive. In this case, the chemical liquid should preferably comprisean anionic surface-active agent. The workpiece should preferablycomprise a semiconductor wafer on which a pattern of STI is formed.

According to an eighth aspect of the present invention, there isprovided a polishing apparatus comprising: a holding device for holdinga workpiece; a polishing table having a fixed abrasive thereon, thefixed abrasive including abrasive particles and a binder; a dressingdevice for generating free abrasive particles from the fixed abrasive;an ejection nozzle for ejecting a fluid onto a surface of the fixedabrasive to remove massive particles, which adversely affect a polishingprocess, from the surface of the fixed abrasive; a controller foradjusting a relative speed between the polishing table and the holdingdevice; and a controller for adjusting a pressing force produced betweenthe polishing table and the holding device.

The above and other objects, features, and advantages of the presentinvention will be apparent from the following description when taken inconjunction with the accompanying drawings which illustrate preferredembodiments of the present invention by way of example.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view showing an entire arrangement of a polishingapparatus according to an embodiment of the present invention;

FIG. 2 is a front view of a polishing section of the polishing apparatusshown in FIG. 1;

FIG. 3 is a schematic side view showing a cleaning unit of the polishingapparatus shown in FIG. 1;

FIG. 4 is a vertical cross-sectional view of a polishing table of apolishing apparatus according to another embodiment of the presentinvention;

FIG. 5A is a cross-sectional view taken along a line P-P of FIG. 4; and

FIG. 5B is a cross-sectional view taken along a line X-X of FIG. 5A.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A polishing apparatus according to embodiments of the present inventionwill be described below with reference to the drawings. FIG. 1 is a planview showing a whole arrangement of a polishing apparatus according toan embodiment of the present invention.

As shown in FIG. 1, the polishing apparatus comprises four load/unloadstages 2 each for receiving a wafer cassette 1 which accommodates aplurality of workpieces such as semiconductor wafers. Each of theload/unload stages 2 may have a mechanism for lifting and lowering thewafer cassette 1. The polishing apparatus has a transfer robot 4provided on rails 3 so that the transfer robot 4 can move along therails 3 to access respective wafer cassettes 1 at respective load/unloadstages 2.

The transfer robot 4 has upper and lower hands. The lower hand of thetransfer robot 4 is a vacuum attraction-type hand for holding asemiconductor wafer under vacuum, and used only for removing a waferfrom a wafer cassette 1. The vacuum attraction-type hand can hold andtransport the semiconductor wafer even if the semiconductor wafer is notlocated at a normal position in the wafer cassette due to a slightdisplacement. The upper hand of the transfer robot 4 is a recesssupport-type hand for supporting a peripheral edge of a semiconductorwafer via a recess formed in the hand, and used only for returning thewafer to the wafer cassette 1. The recess support-type hand cantransport the semiconductor wafer while keeping the semiconductor waferclean because dust is not collected, unlike the vacuum attraction-typehand. In this manner, since a clean semiconductor wafer which has beencleaned is held by the upper hand, the clean semiconductor wafer is notfurther contaminated.

The polishing apparatus has two cleaning units 5, 6 disposed at anopposite side of the load/unload stages 2 with respect to the rails 3 ofthe transfer robot 4. These cleaning units 5, 6 are used for cleaning asemiconductor wafer. The cleaning units 5, 6 are disposed at positionsaccessible by the hands of the transfer robot 4. Each of the cleaningunits 5, 6 has a spin-dry mechanism for drying a wafer by spinning thewafer at a high speed, and hence two-stage cleaning and three-stagecleaning of a wafer can be performed without replacing any cleaningmodules.

Between the two cleaning units 5 and 6, a wafer station 12 having fourwafer supports 7, 8, 9 and 10 is disposed at a position accessible bythe transfer robot 4. A transfer robot 14 having two hands is disposedat a position where hands of the transfer robot 14 can access thecleaning unit 5 and the three wafer supports 7, 9 and 10. A transferrobot 15 having two hands is disposed at a position where hands of thetransfer robot 15 can access the cleaning unit 6 and the three wafersupports 8, 9 and 10.

The wafer support 7 is used to transfer a wafer between the transferrobot 4 and the transfer robot 14 and has a sensor 16 for detectingexistence of a wafer. The wafer support 8 is used to transfer a waferbetween the transfer robot 4 and the transfer robot 15 and has a sensor17 for detecting existence of a wafer.

The wafer support 9 is used to transfer a wafer from the transfer robot15 to the transfer robot 14, and has a sensor 18 for detecting existenceof a wafer and a rinsing nozzle 20 for supplying a rinsing liquid to thewafer to prevent the wafer from being dried or to rinse the wafer. Thewafer support 10 is used to transfer a wafer from the transfer robot 14to the transfer robot 15, and has a sensor 19 for detecting existence ofa wafer and a rinsing nozzle 21 for supplying a rinsing liquid to thewafer to prevent the wafer from being dried or to rinse the wafer.

The wafer supports 9 and 10 are disposed in a commonwater-scatter-prevention cover which has an opening defined therein fortransferring wafers therethrough. The opening can be opened and closedby a shutter 22. The wafer support 9 is disposed above the wafer support10. Upper wafer support 9 serves to support a wafer which has beencleaned, and lower wafer support 10 serves to support a wafer to becleaned, so that the cleaned wafer is prevented from being contaminatedby rinsing liquid which would otherwise fall thereon. The sensors 16,17, 18 and 19, the rinsing nozzles 20, 21, and the shutter 22 areschematically shown in FIG. 1, and their positions and shapes are notexactly illustrated.

A cleaning unit 24 is disposed at a position adjacent to the cleaningunit 5 and is accessible by the hands of the transfer robot 14, andanother cleaning unit 25 is disposed at a position adjacent to thecleaning unit 6 and is accessible by hands of the transfer robot 15.Each of the cleaning units 24 and 25 is capable of cleaning bothsurfaces of a wafer.

FIG. 3 is a schematic side view showing the cleaning unit 24 or 25. Asshown in FIG. 3, the cleaning unit 24 or 25 comprises a plurality ofrollers 110 for rotating a semiconductor wafer W in a horizontal planewhile holding a peripheral portion of the wafer W, an upper polyvinylalcohol (PVA) sponge 112 a for scrubbing an upper surface of the wafer Win a state such that the PVA sponge 112 a is brought into contact withthe upper surface of the wafer W, a lower PVA sponge 112 b for scrubbinga lower surface of the wafer W in a state such that the PVA sponge 112 bis brought into contact with the lower surface of the wafer W, a upperdilute hydrogen fluoride (DHF) supply nozzle 114 a disposed above thewafer W, and a lower DHF supply nozzle 114 b disposed below the wafer W.The DHF supply nozzles 114 a, 114 b supply dilute hydrogen fluoride(DHF) to the wafer W, respectively. Instead of DHF, ozone water orelectrolytic ion water may be supplied to the wafer W. Since ozone waterand electrolytic ion water do not require specific equipment, they caneasily be used in the cleaning unit.

The transfer robot 14 and the transfer robot 15 have respective twohands which are located in a vertically spaced relationship. Therespective upper hands of the transfer robot 14 and the transfer robot15 are used for transporting a semiconductor wafer that has been cleanedto the cleaning units or the wafer supports of the wafer station 12. Therespective lower hands of the transfer robot 14 and the transfer robot15 are used for transporting a semiconductor wafer that has not cleanedor a semiconductor wafer to be polished. Since the lower hands are usedto transfer a wafer to or from a reversing device, the upper hands arenot contaminated by drops of rinsing liquid which fall from an upperwall of the reversing device.

As shown in FIG. 1, the cleaning units 5, 6, 24 and 25 have shutters 5a, 6 a, 24 a and 25 a for transferring wafers therethrough,respectively. The shutters 5 a, 6 a, 24 a and 25 a are opened only whenwafers are transferred through the shutters 5 a, 6 a, 24 a and 25 a.

The polishing apparatus has a housing 26 for enclosing variouscomponents therein. An interior of the housing 26 is partitioned into aplurality of compartments or sections (including areas A and B) bypartition walls 28, 30, 32, 34 and 36.

Area A in which the load/unload stages 2 and the transfer robot 4 aredisposed, and area B in which the cleaning units 5 and 6 and the wafersupports 7, 8, 9 and 10 are disposed, are partitioned by the partitionwall 28 so that cleanliness of area A and area B can be separated fromeach other. The partition wall 28 has an opening for allowingsemiconductor wafers to pass therethrough, and a shutter 38 is providedat the opening of the partition wall 28. All of the cleaning units 5, 6,24 and 25, the wafer supports 7, 8, 9 and 10 of the wafer station 12,and the transfer robots 14 and 15 are placed in area B. Pressure in areaB is adjusted so as to be lower than pressure in area A.

As shown in FIG. 1, in area C separated from area B by the partitionwall 34, a reversing device 40 for reversing a semiconductor wafer isprovided at a position accessible by the hands of the transfer robot 14.The semiconductor wafer is transferred to the reversing device 40 by thetransfer robot 14. Further, in area C, a reversing device 41 forreversing a semiconductor wafer is provided at a position accessible bythe hands of the transfer robot 15. The semiconductor wafer istransferred to the reversing device 41 by the transfer robot 15. Each ofthe reversing devices 40 and 41 has a chuck mechanism for chucking asemiconductor wafer, a reversing mechanism for reversing thesemiconductor wafer, and a wafer detecting sensor for detecting whetheror not the chuck mechanism chucks the semiconductor wafer.

The partition wall 34 forms a polishing section which is separated fromarea B. The polishing section is further divided into two areas C and Dby the partition wall 36. The partition wall 34 between area B and areasC, D has two openings each for allowing semiconductor wafers to passtherethrough, one of which openings is used for transferring a wafer Wto or from the reversing device 40 and the other of which openings isused for transferring a wafer to or from the reversing device 41.Shutters 42, 43 are respectively provided at the openings of thepartition wall 34.

As shown in FIG. 1, each of areas C and D has two polishing tables andone top ring (holding device) for holding and pressing one semiconductorwafer against the polishing tables to polish the wafer. Specifically,area C has a top ring 44, a polishing table 46 having a large diameter,a polishing table 48 having a small diameter, a polishing liquid supplynozzle 50 for supplying a polishing liquid onto the polishing table 46,an atomizer 52 having a plurality of ejection nozzles (not shown)connected to a nitrogen gas supply source and a liquid supply source, adresser 54 for dressing the polishing table 46, and a dresser 56 fordressing the polishing table 48. The diameter of the polishing surfaceof the large-diameter polishing table 46 is not less than twice thediameter of the semiconductor wafer. The diameter of the polishingsurface of the small-diameter polishing table 48 is larger than thediameter of the semiconductor wafer, and is smaller than twice thediameter of the semiconductor wafer. Similarly, area D has a top ring45, a polishing table 47 having a large diameter, a polishing table 49having a small diameter, a polishing liquid supply nozzle 51 forsupplying a polishing liquid onto the polishing table 47, an atomizer 53having a plurality of ejection nozzles (not shown) connected to anitrogen gas supply source and a liquid supply source, a dresser 55 fordressing the polishing table 47, and a dresser 57 for dressing thepolishing table 49.

The polishing liquid supply nozzles 50, 51 supply polishing liquids,used for a polishing process, and dressing liquids (e.g., water) usedfor a dressing process, onto the polishing tables 46, 47, respectively.The atomizers 52, 53 eject liquids composed of a mixture of nitrogen gaswith pure water or a chemical liquid onto the polishing tables 46, 47,respectively. Nitrogen gas from the nitrogen gas supply source and purewater or a chemical liquid from the liquid supply source are passedthrough a regulator or air operated valve (not shown) to regulatepressure thereof to a predetermined value, and supplied to the ejectionnozzles in the atomizers 52, 53 in a mixed state. The chemical liquidmay comprise a surface-active agent, for example. In this case, theliquid should preferably be ejected from the ejection nozzles of theatomizers 52, 53 toward outer peripheral edges of the polishing tables46, 47. Other inert gases may be used instead of nitrogen gas. Further,the atomizers 52, 53 may eject only a liquid of pure water or a chemicalliquid. The polishing tables 48, 49 may have atomizers as with thepolishing tables 46, 47, respectively. With atomizers for the polishingtables 48, 49, surfaces of the polishing tables 48, 49 can be keptclean.

The mixture of nitrogen gas with pure water or the chemical liquid issupplied in a state of (1) liquid fine particles, (2) solid fineparticles as a result of solidification of the liquid, or (3) gas as aresult of vaporization of the liquid. These states (1), (2) and (3) arereferred to as atomization. In these states, the mixture is ejected fromthe ejection nozzles of the atomizers 52, 53 toward the polishing tables46, 47. For example, pressure or temperature of the nitrogen gas and/orthe pure water or the chemical liquid, or the shape of the nozzlesdetermines which state of the mixed liquid is to be ejected, i.e., theliquid fine particles, the solid fine particles, or gas. Therefore, thestate of the liquid to be ejected can be varied, for example, byproperly adjusting pressure or temperature of the nitrogen gas and/orthe pure water or the chemical liquid with use of a regulator or thelike, or by properly adjusting the shape of the nozzles.

The polishing tables 48, 49 may be replaced with wet-type thicknessmeasuring devices for measuring a thickness of a film formed on a wafer.With such wet-type thickness measuring devices, the thickness of a filmformed on a wafer can be measured immediately after the wafer ispolished, and hence it is possible to further polish the polished waferor to control a polishing process for polishing a subsequent wafer basedon measured results.

As shown in FIGS. 1 and 2, a rotary transporter 60 is disposed below thereversing devices 40, 41 and the top ring 44 (in area C) and the topring 45 (in area D). The rotary transporter 60 serves to transfer wafersbetween the cleaning section (area B) and the polishing section (areasC, D). The rotary transporter 60 has four stages for placing wafers W atequal angular intervals, and can hold a plurality of wafers thereon atthe same time.

A wafer which has been transferred to the reversing device 40 or 41 istransferred to a lifter 62 or 63 disposed below the rotary transporter60 by elevating the lifter 62 or 63 when a center of a stage of therotary transporter 60 is aligned with a center of the wafer held by thereversing device 40 or 41. A wafer which has been transferred to thelifter 62 or 63 is transferred to the rotary transporter 60 by loweringthe lifter 62 or 63. A wafer placed on the stage of the rotarytransporter 60 is transported to a position below the top ring 44 (inarea C) or the top ring 45 (in area D) by rotating the rotarytransporter 60 by an angle of 90°. At this time, the top ring 44 (inarea C) or the top ring 45 (in area D) is positioned above the rotarytransporter 60 beforehand by a swinging motion of these top rings. Awafer held on the stage of the rotary transporter 60 is transferred tothe top ring 44 or 45 by elevating a pusher 64 or 65 disposed below therotary transporter 60 when a center of the top ring 44 or 45 is alignedwith a center of the wafer.

Next, the polishing section (areas C, D) will be described below.Although only area C will be described below, the following descriptioncan be applied to area D. FIG. 2 shows a relationship between the topring 44 and the polishing tables 46, 48 in area C.

As shown in FIG. 2, the top ring 44 is supported from a top ring head 72by a top ring drive shaft 70 which is rotatable. The top ring head 72 issupported by a support shaft 74 which can angularly be positioned, andthe top ring 44 can access the polishing tables 46 and 48. The dresser54 is supported from a dresser head 78 by a dresser drive shaft 76 whichis rotatable. The dresser head 78 is supported by a support shaft 80which can angularly be positioned, and the dresser 54 can be movedbetween a standby position and a dressing position above the polishingtable 46. The dresser 56 is similarly supported from a dresser head 84by a dresser drive shaft 82 which is rotatable. The dresser head 84 issupported by a support shaft 86 which can angularly be positioned, andthe dresser 56 can be moved between a standby position and a dressingposition above the polishing table 48. The dressers 54, 56 comprisediamond dressers having diamond particles electrodeposited thereon,respectively, for example.

The large-diameter polishing table 46 has an upper surface composed of afixed abrasive 46 a having abrasive particles and pores or a pore agent,which are fixed by a binder (resin). The upper surface of the fixedabrasive 46 a serves as a polishing surface for polishing asemiconductor wafer held by the top ring 44.Methyl-methacrylate-butadiene-styrene (MBS) resin is used as a binder.MBS resin is a copolymer base on materials of methyl-methacrylate,butadiene, and styrene, and has generally been used as a modifier formodifying shock resistance of vinyl chloride resin or acrylic resin. MBSis a core-shell type thermoplastic resin having a core of a rubber layercomposed of a copolymer (SBR) containing butadiene and styrene, and ashell of a copolymer (MS) containing methyl-methacrylate and styrene.MBS resin may have a core of polybutadiene group rubber or polyacrylicester group rubber, instead of the core of the copolymer (SBR) ofbutadiene and styrene. Further, the binder may comprise a resin in whichan elastomer such as EPR, butadiene rubber, or ethylene-propylene rubberis dispersed, or a core-shell type resin having a core of an elastomer.

The small-diameter polishing table 48 has an upper surface composed of asoft nonwoven fabric. The upper surface of the nonwoven fabric serves asa cleaning surface for cleaning a semiconductor wafer after a polishingprocess to remove abrasive particles attached to a surface of the wafer.

Next, a polishing process for polishing a semiconductor wafer with theuse of a polishing apparatus according to the present invention will bedescribed below. Although a polishing process only in area C will bedescribed below, the following description can be applied to a polishingprocess in area D.

First, a polishing process for polishing a semiconductor wafer on whicha pattern of interlayer dielectrics (ILD) is formed will be describedbelow. For example, the following dressing process (ex-situ dressingprocess) is performed in a case of polishing a wafer which is likely tobe carved and is polished at a high polishing rate.

1) Ex-situ dressing process

The polishing table 46 is rotated at a rotational speed of 25revolutions per minute, and the dresser 54 is rotated at a rotationalspeed of 10 revolutions per minute. The dresser 54 is pressed againstthe polishing table 46 under a pressing force of 40 N. The fixedabrasive 46 a mounted on the polishing table 46 is dressed for 17seconds, for example. At this time, the atomizer 52 ejects a liquidcomposed of a mixture of deionized water (DIW) having a flow rate of1000 cm³/min and nitrogen gas having a pressure of 0.15 MPa onto thefixed abrasive 46 a. Thus, a dressing process is performed on the fixedabrasive 46 a while atomized liquid is being ejected onto the fixedabrasive 46 a. Even if massive particles, which cause scratches on asurface of the wafer, are produced on a surface of the fixed abrasive 46a by performing the dressing process, the atomized liquid which isejected onto the fixed abrasive 46 a can remove the massive particlesfrom the surface of the fixed abrasive 46 a to prevent the wafer frombeing scratched. The atomized liquid may be ejected after the dressingprocess.

If pressure of the nitrogen gas is higher than 0.5 MPa, then a polishingrate is lowered. Therefore, it is desirable that a flow rate of DIW iswithin a range of from 200 to 5000 cm³/min, and pressure of the nitrogengas is within a range of from 0.05 to 0.5 MPa. More preferably, the flowrate of DIW is about 1000 cm³/min and the pressure of the nitrogen gasis about 0.15 MPa. Further, when the ejection nozzles of the atomizer 52are directed toward outer peripheral edges of the fixed abrasive 46 a,the aforementioned massive particles can efficiently be removed from thesurface of the fixed abrasive 46 a.

2) Polishing process

The polishing table 46 is rotated at a rotational speed of 10revolutions per minute, and the top ring 44 is rotated at a rotationalspeed of 16 revolutions per minute. A wafer is pressed against thepolishing table 46 under a surface pressure of 35 kPa. The wafer ispolished for 120 seconds, for example. At this time, the polishingliquid supply nozzle 50 supplies a polishing liquid and DIW or achemical liquid (surface-active agent) onto the fixed abrasive 46 a at aflow rate of 200 cm³/min. Although surface pressure of the wafer is 35kPa in this example, it should preferably be 35 kPa or less in order toprevent scratches of the wafer. Rotational speeds of the polishing table46 and the top ring 44 during the polishing process may be higher thanthe above rotational speeds, respectively. For example, the polishingtable 46 is rotated at a rotational speed of 60 revolutions per minute,and the top ring 44 is rotated at a rotational speed of 61 revolutionsper minute. In this case, a polishing rate is lowered, but the wafer isprevented from being scratched. The polishing process may be performedwhile the atomized liquid is being ejected onto the fixed abrasive 46 a.In this case, the atomized liquid which is ejected onto the fixedabrasive 46 a can remove massive particles produced by sliding frictionbetween the wafer and the fixed abrasive 46 a.

3) Water-polishing process

After the polishing process, the polishing table 46 is rotated at arotational speed of 50 revolutions per minute, and the top ring 44 isrotated at a rotational speed of 40 revolutions per minute. The wafer ispressed against the polishing table 46 under a surface pressure of 10kPa. The wafer is water-polished for 10 seconds, for example. At thistime, the polishing liquid supply nozzle 50 supplies a polishing liquidor DIW onto the fixed abrasive 46 a at a flow rate of 1000 cm³/min.

In the water-polishing process, the rotational speed of the polishingtable 46 (50 revolutions per minute) is larger than that during thepolishing process (10 revolutions per minute). Thus, a relativerotational speed between the polishing surface and the semiconductorwafer is increased to enhance an effect of cleaning and removingabrasive particles attached to the surface of the semiconductor wafer.Further, surface pressure of the wafer (10 kPa) is lower than thatduring the polishing process (35 kPa), so that the wafer is preventedfrom being further polished during the water-polishing process.Additionally, the amount of liquid supplied to the fixed abrasive 46 ais larger than that during the polishing process, and surface pressureof the wafer is lower than that during the polishing process. As aresult, a liquid film between the semiconductor wafer and the polishingsurface is thickened, and surface tension therebetween is reduced.Therefore, the semiconductor wafer can easily be separated or removedfrom the surface of polishing table 46.

4) Overhanging process

After the water-polishing process, the polishing table 46 is rotated ata rotational speed of 5 revolutions per minute, and the top ring 44 isrotated at a rotational speed of 10 revolutions per minute. The top ring44 is moved horizontally along the surface of the polishing table 46 byperforming a swinging motion of the top ring head 72. Surface pressureof the wafer at this time is adjusted to be lower than 10 kPa. Thepolishing liquid supply nozzle 50 supplies a polishing liquid or DIWonto the fixed abrasive 46 a at a flow rate of 1000 cm³/min. Then, thetop ring head 72 is stopped at an overhanging position where a portionof the wafer is exposed beyond an outer peripheral edge of the polishingtable 46. The top ring 44 is lifted at the overhanging position toseparate or remove the wafer from the surface of the fixed abrasive 46a. This overhanging action allows surface tension of liquid which isproduced between the wafer and the polishing table to be reduced. Thus,it is possible to eliminate an undesired force between the wafer and thepolishing table and to reliably separate or remove the wafer from thepolishing table. Therefore, the wafer can accurately be transferred to asubsequent process. During the overhanging process, rotational speed ofthe polishing table 46 is lowered from 10 revolutions per minute to 5revolutions per minute, so that the semiconductor wafer can stably beseparated or removed from the polishing table 46 without scratches orunevenly polished portions. The top ring 44 may be lifted at a polishingposition, and the semiconductor wafer may be removed from the surface ofthe fixed abrasive 46 a without the above overhanging action. In such acase, when the fixed abrasive 46 a has a plurality of grooves formed inthe surface thereof, the wafer can easily be removed from the surface ofthe fixed abrasive 46 a.

As described above, the above ex-situ dressing process is performed in acase of polishing a wafer which is likely to be carved and is polishedat a high polishing rate. On the other hand, in a case of polishing asemiconductor wafer which has a pattern of ILD, is unlikely to becarved, and is polished at a low polishing rate, the following dressingprocess (in-situ dressing process) is performed. The same or likeprocesses as in the aforementioned example will not be described belowrepetitively.

1) In-situ dressing and polishing process

The polishing table 46 is rotated at a rotational speed of 10revolutions per minute, the dresser 54 is rotated at a rotational speedof 10 revolutions per minute, and the top ring 44 is rotated at arotational speed of 26 revolutions per minute. A wafer is pressedagainst the polishing table 46 under a surface pressure of 50 kPa. Thewafer is polished for 60 seconds, for example. At this time, thepolishing liquid supply nozzle 50 supplies a polishing liquid and DIW ora chemical liquid (surface-active agent) onto the fixed abrasive 46 a ata flow rate of 200 cm³/min, and the atomizer 52 ejects a liquid composedof a mixture of DIW having a flow rate of 1000 cm³/min and nitrogen gashaving a pressure of 0.15 MPa onto the fixed abrasive 46 a. Thereafter,a dressing process of the dresser 54 is stopped. Rotational speed of thetop ring 44 is lowered to 16 revolutions per minute, and surfacepressure of the wafer is lowered to 35 kPa. A polishing process of thesemiconductor wafer is continued for 120 seconds, for example.Specifically, the semiconductor wafer is polished while performing thedressing process (in-situ dressing process) until immediately beforeconvexes formed on a surface of the wafer are planarized, i.e., untilremaining steps on the wafer become 100 to 300 Å. Thereafter, thepolishing process is continued while stopping the dressing process untilthe convexes are planarized. Even if massive particles are produced onthe surface of the fixed abrasive 46 a by the dressing process of thedresser 54 and scratch the surface of the semiconductor wafer, thescratches can gradually be shallowed by continuously polishing the waferand can finally be eliminated. Although surface pressure of the wafer islowered to 35 kPa in this example, it should preferably be lowered to 35kPa or less in order to prevent scratches of the wafer. The polishingprocess may continuously be performed after the dressing process isstopped while the atomized liquid is being ejected onto the fixedabrasive 46 a. In this case, the atomized liquid which is ejected ontothe fixed abrasive 46 a can remove massive particles produced by slidingfriction between the wafer and the fixed abrasive 46 a.

2) Water-polishing process

After the polishing process, the polishing table 46 is rotated at arotational speed of 50 revolutions per minute, and the top ring 44 isrotated at a rotational speed of 40 revolutions per minute. The wafer ispressed against the polishing table 46 under a surface pressure of 10kPa. The wafer is water-polished for preferably not less than 5 seconds,for example, 10 seconds. At this time, the polishing liquid supplynozzle 50 supplies a polishing liquid or DIW onto the fixed abrasive 46a at a flow rate of 1000 cm³/min.

3) Overhanging process

After the water-polishing process, the polishing table 46 is rotated ata rotational speed of 5 revolutions per minute, and the top ring 44 isrotated at a rotational speed of 10 revolutions per minute. The top ring44 is moved horizontally along the surface of the polishing table 46 byperforming a swinging motion of the top ring head 72. Surface pressureof the wafer at this time is adjusted to be lower than 10 kPa. Thepolishing liquid supply nozzle 50 supplies a polishing liquid or DIWonto the fixed abrasive 46 a at a flow rate of 1000 cm³/min. Then, thetop ring head 72 is stopped at an overhanging position where a portionof the wafer is exposed beyond an outer peripheral edge of the polishingtable 46. The top ring 44 is lifted at the overhanging position toseparate or remove the wafer from the surface of the fixed abrasive 46a. The top ring 44 may be lifted at a polishing position, and thesemiconductor wafer may be removed from the surface of the fixedabrasive 46 a without the overhanging action. In such a case, when thefixed abrasive 46 a has a plurality of grooves formed in the surfacethereof, the wafer can easily be removed from the surface of the fixedabrasive 46 a.

Next, a polishing process for polishing a semiconductor wafer on which apattern of shallow trench isolations (STI) is formed will be describedbelow. As with ILD, the following ex-situ dressing process is performedin a case of polishing a wafer which is likely to be carved and ispolished at a high polishing rate.

1) Ex-situ dressing process

The polishing table 46 is rotated at a rotational speed of 25revolutions per minute, and the dresser 54 is rotated at a rotationalspeed of 10 revolutions per minute. The dresser 54 is pressed againstthe polishing table 46 under a pressing force of 40 N. The fixedabrasive 46 a mounted on the polishing table 46 is dressed for 17seconds, for example. At this time, the atomizer 52 ejects a liquidcomposed of a mixture of DIW having a flow rate of 1000 cm³/min andnitrogen gas having a pressure of 0.15 MPa onto the fixed abrasive 46 a.

2) Polishing process

The polishing table 46 is rotated at a rotational speed of 10revolutions per minute, and the top ring 44 is rotated at a rotationalspeed of 16 revolutions per minute. The wafer is pressed against thepolishing table 46 under a surface pressure of 35 kPa. The wafer ispolished for 120 seconds, for example. At this time, the polishingliquid supply nozzle 50 supplies a polishing liquid and DIW or achemical liquid (anionic surface-active agent) onto the fixed abrasive46 a at a flow rate of 200 cm³/min. Although surface pressure of thewafer is 35 kPa in this example, it should preferably be 35 kPa or lessin order to prevent scratches of the wafer. Rotational speeds of thepolishing table 46 and the top ring 44 during a polishing process may behigher than the above rotational speeds, respectively. For example, thepolishing table 46 is rotated at a rotational speed of 60 revolutionsper minute, and the top ring 44 is rotated at a rotational speed of 61revolutions per minute. In this case, a polishing rate is lowered, butthe wafer is prevented from being scratched. The polishing process maybe performed while the atomized liquid is being ejected onto the fixedabrasive 46 a. In this case, the atomized liquid which is ejected ontothe fixed abrasive 46 a can remove massive particles produced by slidingfriction between the wafer and the fixed abrasive 46 a.

During a polishing process of an STI wafer, when an anionicsurface-active agent is supplied as a polishing liquid, polishing of anitride film is prevented, so that the nitride film serves as a stopperfor enhancing uniformity within the wafer. In this case, the same kindof chemical liquid as the polishing liquid (e.g., anionic surface-activeagent) should preferably be added to liquid used for the atomizer duringthe polishing process. If concentration of the surface-active agent inthe liquid on the polishing surface is reduced by the atomizer, then thenitride film is polished. Therefore, it is important not to changeconcentration of a liquid which is brought into contact with the waferduring the polishing process.

When the STI wafer is polished, two types of films, i.e., an oxide filmand a nitride film, are exposed on the same surface of the wafer. Aconventional polishing method using an elastic soft pad has a problem ofdishing of the oxide film at trenches. The present invention, with ahard fixed abrasive, is particularly effective in planarization of STIwafers in which different kinds of materials are exposed.

3) Water-polishing process

After the polishing process, the polishing table 46 is rotated at arotational speed of 50 revolutions per minute, and the top ring 44 isrotated at a rotational speed of 40 revolutions per minute. The wafer ispressed against the polishing table 46 under a surface pressure of 10kPa. The wafer is water-polished for preferably not less than 5 seconds,for example, 10 seconds. At this time, the polishing liquid supplynozzle 50 supplies a polishing liquid or DIW onto the fixed abrasive 46a at a flow rate of 1000 cm³/min.

4) Overhanging process

After the water-polishing process, the polishing table 46 is rotated ata rotational speed of 5 revolutions per minute, and the top ring 44 isrotated at a rotational speed of 10 revolutions per minute. The top ring44 is moved horizontally along the surface of the polishing table 46 byperforming a swinging motion of the top ring head 72. Surface pressureof the wafer at this time is adjusted to be lower than 10 kPa. Thepolishing liquid supply nozzle 50 supplies a polishing liquid or DIWonto the fixed abrasive 46 a at a flow rate of 1000 cm³/min. Then, thetop ring head 72 is stopped at an overhanging position where a portionof the wafer is exposed beyond an outer peripheral edge of the polishingtable 46. The top ring 44 is lifted at the overhanging position toseparate or remove the wafer from the surface of the fixed abrasive 46a. The top ring 44 may be lifted at a polishing position, and thesemiconductor wafer may be removed from the surface of the fixedabrasive 46 a without the overhanging action. In such a case, when thefixed abrasive 46 a has a plurality of grooves formed in the surfacethereof, the wafer can easily be removed from the surface of the fixedabrasive 46 a.

As described above, the above ex-situ dressing process is performed in acase of polishing a wafer which is likely to be carved and is polishedat a high polishing rate. On the other hand, in a case of polishing asemiconductor wafer which has a pattern of STI, is unlikely to becarved, and is polished at a low polishing rate, the following in-situdressing process is performed, as with ILD.

1) In-situ dressing and polishing process

The polishing table 46 is rotated at a rotational speed of 10revolutions per minute, the dresser 54 is rotated at a rotational speedof 10 revolutions per minute, and the top ring 44 is rotated at arotational speed of 26 revolutions per minute. The wafer is pressedagainst the polishing table 46 under a surface pressure of 50 kPa. Thewafer is polished for 60 seconds, for example. At this time, thepolishing liquid supply nozzle 50 supplies a polishing liquid and DIW ora chemical liquid (anionic surface-active agent) onto the fixed abrasive46 a at a flow rate of 200 cm³/min, and the atomizer 52 ejects a liquidcomposed of a mixture of DIW having a flow rate of 1000 cm³/min andnitrogen gas having a pressure of 0.15 MPa onto the fixed abrasive 46 a.Thereafter, a dressing process of the dresser 54 is stopped. Rotationalspeed of the top ring 44 is lowered to 16 revolutions per minute, andthe surface pressure of the wafer is lowered to 35 kPa. A polishingprocess of the semiconductor wafer is continued for 120 seconds, forexample. Specifically, the semiconductor wafer is polished whileperforming the dressing process (in-situ dressing process) untilimmediately before a surface of the semiconductor wafer is planarized sothat a nitride film is exposed, i.e., until a thickness of a remainingoxide film on the nitride film becomes about 500 Å. Thereafter, thepolishing process is continued while stopping the dressing process untilthe nitride film is exposed. Even if massive particles are produced onthe surface of the fixed abrasive 46 a by the dressing process of thedresser 54 and scratch the surface of the semiconductor wafer, thescratches can gradually be shallowed by continuously polishing the waferand can finally be eliminated. Although surface pressure of the wafer islowered to 35 kPa in this example, it should preferably be lowered to 35kPa or less in order to prevent scratches of the wafer.

2) Water-polishing process

After the polishing process, the polishing table 46 is rotated at arotational speed of 50 revolutions per minute, and the top ring 44 isrotated at a rotational speed of 40 revolutions per minute. The wafer ispressed against the polishing table 46 under a surface pressure of 10kPa. The wafer is water-polished for preferably not less than 5 seconds,for example, 10 seconds. At this time, the polishing liquid supplynozzle 50 supplies a polishing liquid or DIW onto the fixed abrasive 46a at a flow rate of 1000 cm³/min.

3) Overhanging process

After the water-polishing process, the polishing table 46 is rotated ata rotational speed of 5 revolutions per minute, and the top ring 44 isrotated at a rotational speed of 10 revolutions per minute. The top ring44 is moved horizontally along the surface of the polishing table 46 byperforming a swinging motion of the top ring head 72. Surface pressureof the wafer at this time is adjusted to be lower than 10 kPa. Thepolishing liquid supply nozzle 50 supplies a polishing liquid or DIWonto the fixed abrasive 46 a at a flow rate of 1000 cm³/min. Then, thetop ring head 72 is stopped at an overhanging position where a portionof the wafer is exposed beyond an outer peripheral edge of the polishingtable 46. The top ring 44 is lifted at the overhanging position toseparate or remove the wafer from the surface of the fixed abrasive 46a. The top ring 44 may be lifted at a polishing position, and thesemiconductor wafer may be removed from the surface of the fixedabrasive 46 a without the overhanging action. In such a case, when thefixed abrasive 46 a has a plurality of grooves formed in the surfacethereof, the wafer can easily be removed from the surface of the fixedabrasive 46 a.

The semiconductor wafer thus polished with the fixed abrasive 46 a istransferred to the small-diameter polishing table 48, in which a buffcleaning process is performed. Specifically, while the top ring 44 andthe polishing table 48 are respectively rotated independently of eachother, the polished semiconductor wafer held by the top ring 44 ispressed against the soft nonwoven fabric on the polishing table 48. Atthis time, a liquid containing no abrasive particles, such as pure wateror alkali liquid, is supplied onto the nonwoven fabric from a cleaningliquid supply nozzle (not shown). The alkali liquid should preferablycomprise an alkali liquid having a pH of 9 or larger, or an alkaliliquid containing TMAH. With this buff cleaning process, abrasiveparticles attached to a surface of the polished semiconductor wafer caneffectively be removed from the surface of the wafer.

Instead of the above buff cleaning process, a DHF cleaning process ofthe semiconductor wafer may be performed in the cleaning unit 23 or 25.In this case, DHF supply nozzles 114 a, 114 b eject a DHF liquid of 0.5%toward the wafer for about 30 seconds, for example (see FIG. 3). Withthe DHF cleaning process, an oxide film on the surface of the polishedsemiconductor wafer is dissolved and removed by the DHF liquid, andabrasive particles which have been attached to the surface of the oxidefilm are simultaneously removed. In this case, it is desirable to use aDHF liquid having a concentration of 0.1% or higher. When the wafer isscrubbed with the PVA sponges 112 a, 112 b, as shown in FIG. 3, theabrasive particles can be removed more effectively. Ozone water orelectrolytic ion water may be used instead of a DHF liquid. After thebuff cleaning process or the DHF cleaning process, the surface of thesemiconductor wafer may be cleaned with a pencil-type sponge or thelike. Particularly, when abrasive particles used in the fixed abrasivecomprise cerium oxide, abrasive particles of cerium oxide are likely tobe attached to the surface of the wafer. Therefore, the buff cleaningprocess or the DHF cleaning process can achieve a clean wafer after thepolishing process.

After the polishing process with the fixed abrasive 46 a, a finalpolishing process may be performed on the semiconductor wafer. Thisfinal polishing process may be performed using either of the polishingtable 46 and the polishing table 48. In either case, the final polishingprocess is performed with use of a polishing liquid containing abrasiveparticles, and a water-polishing process and cleaning process (buffcleaning process or DHF cleaning process) are performed after the finalpolishing process.

In the present embodiment, the large-diameter polishing table 46 has thefixed abrasive thereon, and the small-diameter polishing table 48 hasthe polishing pad (nonwoven fabric) thereon. A wafer is polished withthe large-diameter polishing table 46 and then polished with thesmall-diameter polishing table 48. Thus, a two-stage polishing processis performed in the present embodiment. However, the present inventionis not limited to such a two-stage polishing process.

Generally, in a case of a polishing table having a small diameter whichprovides a small relative speed between a polishing surface and asemiconductor wafer, a sufficient polishing rate cannot be achieved.Therefore, when it is necessary to maintain a polishing rate of acertain level, a primary polishing process is generally performed with apolishing table having a large diameter which can provide a largerelative speed between a polishing surface and the semiconductor wafer.However, when the aforementioned fixed abrasive is used during thepolishing process, a polishing rate of a certain level can be maintainedeven if a relative speed between the polishing surface and thesemiconductor wafer is small. Therefore, a primary polishing process canbe performed with the small-diameter polishing table 48. From this pointof view, the small-diameter polishing table 48 may have a fixed abrasiveand the large-diameter polishing table 46 may have a polishing pad(polishing cloth), for example. In this case, a semiconductor wafer maybe polished with the small-diameter polishing table 48 and then polishedwith the large-diameter polishing table 46.

A fixed abrasive is more expensive than a polishing pad, and a pricethereof becomes higher in proportion to a diameter thereof. The lifetimeof a polishing pad is shorter than that of a fixed abrasive. Therefore,when a polishing pad is applied to a large-diameter polishing tablewhich can disperse frequency of contact with a wafer to prolong thelifetime of the polishing pad, frequency of maintenance is lengthened toimprove productivity. As described above, a fixed abrasive which is moreexpensive than a polishing pad and is difficult to be formed is used forthe small-diameter polishing table 48, and a polishing pad of which thelifetime is shorter than that of a fixed abrasive is used for thelarge-diameter polishing table 46. A wafer is roughly polished with thesmall-diameter polishing table 48 and then finally polished with thelarge-diameter polishing table 46. With this process, running cost canbe reduced, and it is easy to perform maintenance of the polishingapparatus.

From a viewpoint of planarization and cost savings, a polishing toolused for the small-diameter polishing table 48 may comprise anypolishing tool other than the fixed abrasive. For example, a hard padwhich is not a fixed abrasive may be used for the small-diameterpolishing table 48, and a wafer may be polished while supplying achemical liquid having an etching effect or the like, such as anoxidizing agent for polishing a Cu film, an etching agent, or anoxidation inhibitor. Particularly, when a chemical liquid such as anetching agent is supplied for a metallic film formed on a semiconductorwafer, the chemical liquid degrades a surface of the metallic film.Therefore, a removal process of a surface of the semiconductor wafer canbe performed without abrasive particles by mechanical effects of slidingcontact between the hard pad and the semiconductor wafer. Although thechemical liquid should preferably contain no abrasive particles, it maycontain abrasive particles. Further, pure water containing abrasiveparticles may be used as the chemical liquid. In these cases, it ispossible to achieve a high level of planarization and a high polishingrate as with a fixed abrasive.

When a hard pad is used for polishing a wafer, fine scratches are likelyto be produced on a surface of the wafer, and hence it is necessary toperform a final polishing process thereafter. Further, when the wafer isplanarized, a polishing rate is considerably lowered. Therefore, ittakes much time to further polish the wafer to a predetermined filmthickness after the wafer is planarized. Consequently, it is desirablethat after a polishing process with the small-diameter polishing table48, a final polishing process is subsequently performed with thelarger-diameter polishing table 46. In this case, from a viewpoint ofachieving a high polishing rate and removing scratches from the wafer,it is desirable that a soft pad is mounted on the large-diameterpolishing table 46, and the final polishing process is performed whilesupplying slurry to the soft pad.

In some cases, a thickness of a film formed on a surface of asemiconductor wafer is different between a central portion and aperipheral portion of the semiconductor wafer. For example, a film onthe surface is thin at a central portion of a semiconductor wafer,gradually becomes thicker toward a peripheral portion of thesemiconductor wafer, and becomes thin at an outermost peripheral portionof the semiconductor wafer. In order to planarize such a semiconductorwafer when the semiconductor wafer is polished with a soft polishing padwhile supplying slurry thereto, it is necessary to control the top ringso as to follow a varying film profile, which is too complicated to beachieved. When such a semiconductor wafer is polished with a polishingsurface of a fixed abrasive or a hard pad that is not a fixed abrasivewhile supplying a chemical liquid having an etching effect, it ispossible to achieve planarization of such a semiconductor wafer. Such apolishing process can be applied to a case where semiconductor chipshave differences of densities of patterns and pitches of convexities onsurfaces thereof.

In the above embodiment, a polishing tool mounted on the large-diameterpolishing table 46 is not limited to the polishing pad, and may comprisea fixed abrasive. In a case where a fixed abrasive is mounted on thelarge-diameter polishing table 46, a semiconductor wafer is planarizedwith the small-diameter polishing table 48 and then finally polishedwith the large-diameter polishing table 46 while supplying pure water ora chemical liquid containing no abrasive particles.

The polishing tables are not limited to the polishing tables shown inFIG. 2. For example, the polishing tables 46, 47, 48 and 49 may comprisea scroll-type polishing table as shown in FIGS. 4, 5A and 5B. Such ascroll-type polishing table will be described below. FIG. 4 is avertical cross-sectional view showing a scroll-type polishing table in apolishing apparatus according to another embodiment of the presentinvention. FIG. 5A is a cross-sectional view taken along a line P-P ofFIG. 4, and FIG. 5B is a cross-sectional view taken along a line X-X ofFIG. 5A.

As shown in FIGS. 4, 5A and 5B, the scroll-type polishing table has anupper flange 251 of a motor 250, and a hollow shaft 252 connected to theupper flange 251 by bolts. A set ring 254 is supported by an upperportion of the shaft 252 through a bearing 253. A table 259 is fixed tothe set ring 254, and polishing table 255 is fixed to the table 259 bybolts 290. The polishing table 255 may comprise a fixed abrasiveentirely, or may comprise a plate made of a corrosion-resistant metalsuch as stainless steel and a polishing pad attached to the plate. Whenusing a fixed abrasive or a polishing pad, the polishing table 255 mayhave a flat upper surface or a slightly convex or concave upper surface.An outer diameter of the polishing table 255 is set to be equal to orlarger than a diameter of a wafer plus a distance “2e”. Specifically,the diameter of the polishing table 255 is arranged such that when thepolishing table 255 makes a translational motion, the wafer does notproject from an outer periphery of the polishing table 255.

The set ring 254 has three or more supporting portions 258 in acircumferential direction for supporting the table 259. A plurality ofrecesses 260, 261 are formed, at positions corresponding to an uppersurface of the supporting portions 258, in the set ring 254 and an upperend of a cylindrical member, at angularly equal intervals in acircumferential direction. Bearings 262, 263 are mounted in the recesses260, 261. As shown in FIGS. 4, 5A and 5B, a support member 266 havingtwo shafts 264, 265 is supported by the bearings 262, 263. Central axesof the shafts 264, 265 are spaced from each other by a distance “e”.Specifically, the two shafts 264, 265 are inserted into the bearings262, 263, respectively. Thus, the polishing table 255 makes atranslational motion along a circle having a radius “e” by driving themotor 250.

Further, a center of the shaft 252 is off-centered by distance “e” froma center of the motor 250. A balancer 267 is fixed to the shaft 252 forproviding a balance to load caused by eccentricity.

A polishing liquid is supplied onto the polishing table 255 through aninterior of the motor 250 and the shaft 252, a through-hole 257 formedin a central portion of the table 259, and a coupling 291. The suppliedpolishing liquid is once stored in a space 256 formed between thepolishing table 255 and the table 259, and then supplied to an uppersurface of the polishing table 255 through a plurality of through-holes268 formed in the polishing table 255 so as to be brought into directcontact with a wafer. The number and positions of the through-holes 268can be selected depending on the kind of process being performed. In acase where a polishing pad is attached to the polishing table 255, thepolishing pad has through-holes at positions corresponding to thepositions of the through-holes 268. In a case where the polishing table255 is made of a fixed abrasive in its entirety, the upper surface ofthe polishing table 255 may have lattice-like grooves, spiral grooves,or radial grooves so as to communicate with the through-holes 268.

The supplied polishing liquid may be selected from pure water, chemicalliquids, or slurry. More than one kind of polishing liquid can besupplied simultaneously, alternatively, or sequentially, as needed.

In order to protect a mechanism for performing a translational motion,from a polishing liquid used for polishing, a flinger or a thrower 269is attached to the table 255, and forms a labyrinth mechanism togetherwith a trough 270.

With the above structure, when the motor 250 is driven, the polishingtable 255 makes a circulatory translational motion (scroll motion) and asemiconductor wafer held by top ring 44 is pressed against a polishingsurface of the polishing table 255. The semiconductor wafer is polishedby polishing liquid supplied through the through-hole 257, the space 256and the through-holes 268. The semiconductor wafer is polished byrelative circulatory translational motion, having a radius “e” , betweena polishing surface of the polishing table 255 and the wafer. Thus, thesemiconductor wafer is uniformly polished over the entire surfacethereof. If a surface, to be polished, of the semiconductor wafer andthe polishing surface have the same positional relationship, then apolished semiconductor wafer is affected by a local difference in thepolishing surface. In order to eliminate this influence, the top ring 44is rotated at a low speed to prevent the semiconductor wafer from beingpolished at the same area on the polishing surface.

In the above embodiments, the dressers 54, 55, 56 and 57 comprisedressers having diamond particles electrodeposited thereon as mechanicaldressing tools. However, various dressing tools may be used forpromoting self-generation of the abrasive particles from the fixedabrasive to the polishing surface, depending on composition andcharacteristics of the fixed abrasive. For example, in a case of a fixedabrasive in which abrasive particles are likely to be self-generated,dressing tools may comprise a brush dresser having a nylon brush or thelike, an optical dresser for modifying a surface of the fixed abrasiveby light emission to promote self-generation of the abrasive particles,an ultrasonic dresser for vibrating a liquid on a polishing surface byultrasonic vibration to promote self-generation of the abrasiveparticles, or a chemical liquid dresser for dissolving or modifyingbinder of the fixed abrasive by a chemical liquid to promoteself-generation of the abrasive particles.

Although certain preferred embodiments of the present invention havebeen shown and described in detail, it should be understood that variouschanges and modifications may be made therein without departing from thescope of the appended claims.

1. A method comprising: polishing a workpiece by pressing said workpieceagainst a fixed abrasive and bringing said workpiece into slidingcontact with said fixed abrasive; and then water-polishing saidworkpiece while supplying pure water to said fixed abrasive, whereinsurface pressure of said workpiece during the water-polishing is set tobe smaller than that during the polishing using said fixed abrasive. 2.The method according to claim 1, wherein the water-polishing comprisesincreasing a rotational speed of a polishing table.
 3. The methodaccording to claim 1, wherein the water-polishing comprises supplyingsaid pure water at a flow rate larger than a flow rate of a polishingliquid supplied during the polishing using said fixed abrasive.
 4. Themethod ac cording to claim 1, further comprising: removing saidworkpiece from said fixed abrasive after the water-polishing, whereinthe removing comprises reducing a rotational speed of a polishing tableas compared to a rotational speed of the polishing table during thepolishing using said fixed abrasive.
 5. A method comprising: polishing aworkpiece with a fixed abrasive while supplying a chemical liquid to asurface of said fixed abrasive; and simultaneously ejecting at least oneof a liquid containing the chemical liquid, and a fluid composed of amixture of inert gas and the chemical liquid, onto said surface of saidfixed abrasive.
 6. The method according to claim 5, wherein the chemicalliquid comprises an anionic surface-active agent.
 7. The methodaccording to claim 6, wherein the workpiece comprises a semiconductorwafer on which a pattern of STI is formed.
 8. A polishing apparatuscomprising: a holding device for holding a workpiece; a polishing tablehaving a fixed abrasive thereon, said fixed abrasive including abrasiveparticles and a binder; a dressing device for generating free abrasiveparticles from said fixed abrasive; an ejection nozzle for ejecting afluid onto a surface of said fixed abrasive to remove massive particles,which adversely affect a polishing process, from the surface of saidfixed abrasive; a controller for adjusting a relative speed between saidpolishing table and said holding device; and a controller for adjustinga pressing force produced between said polishing table and said holdingdevice.